Wood structural components
Editor's note: This story is adapted from the U.S. Department of Housing and Urban Development's Residential Rehabilitation Inspection Guide, 2000.Click here for other stories in this series.
Parts of this story: Introduction ~~ Seismic and wind resistance ~~Cracking and deterioration of masonry, general issues ~~Masonry foundations & piers~~Above ground masonry walls ~~Chimneys ~~ Wood structural components ~~Iron and steel structural components ~~Concrete structural components
|A familiar sight to owners of old houses. This sagging floor is caused by settlement of a basement support. It may be possible to jack the floor and its adjacent wall into a level position, but this should be done slowly and carefully.|
Wood structural components in small residential buildings are often directly observable only in attics, crawl spaces, or basements. Elsewhere they are concealed by floor, wall, and ceiling materials. Common signs of wood structural problems are sloping or springy floors, wall and ceiling cracks, wall bulges, and sticking doors and windows, although many such problems may be attributable to differential settlement of the foundation or problems with exterior masonry bearing walls.
When failures in wood structural components occur, they usually involve individual wood members and rarely result in the failure of the entire structure. Instead, an elastic adjustment takes place that redistributes stresses to other components in the building.
The four types of problems commonly associated with components in small residential buildings are:
Inspect for these problems as follows:
1. Deflection, warping, and associated problems.
Some deflection of wood structural components or assemblies is common in older buildings and normally can be tolerated, unless it causes loss of bearing or otherwise weakens connections or it opens watertight joints in roofs or other critical locations.
Deflection can be arrested by the addition of supplemental supports or strengthening members. Once permanently deflected, however, a wood structural component cannot be straightened. Warping of individual wood components almost always takes place early in the life of a building. It will usually cause only superficial damage, although connections may be loosened and occasionally there may be a loss of bearing.
Look for the following problems associated with wood structural components:Loss of bearing in beams and joists over foundation walls, piers, or columns due to movements caused by long-term deflection of the wood beams or joists, differential movements of the foundation elements, localized crushing, or wood decay. Check the bearing and connections of all exposed structural elements that are in contact with the foundation and look for symptoms of bearing failure where these elements are concealed, such as bowing or sloping in the floor above, and cracking or tilting of foundation walls, piers, and columns.
Some reasons for column settling
Sagging, sloping, or springing of floors due to foundation settlement, excessive spans, cut or drilled structural elements, overloading, or removal of supporting walls or columns on the floor above or below. Each case must be diagnosed separately.
In older buildings, columns or walls that helped support or stabilize the floor above may have been removed during a previous alteration; conversely, partitions, bathrooms, kitchens, or similar remodelings may have been placed on a floor not designed to support such additional loads. Depending on the circumstances, sagging, sloping, or springing floors may be any-thing from an annoyance to an indication of a potentially serious structural problem.
Check below the floor for adequate supports and bearing and for sound connections between structural elements. Look for signs of supporting walls that have been removed, missing joist hangars, and for inappropriate cuts or holes in joists for plumbing, electric, or HVAC lines or ducts. Also look for signs of insect or fungal attack.
Floor sagging near stairway openings due to gradual deflection of the unsupported floor framing. This is a common problem in older houses and usually does not present a structural problem. Correction, if desired, will be difficult since the whole structural assembly surrounding the stair has deformed. Look for signs that a supporting wall below the opening has been removed. Where this has occurred, structural modification or the addition of a supporting column may be required.
Roll a marble or similar heavy round object over suspect floor areas to determine the direction and degree of slope, if any. A carpenters level also can be used. For large or complex areas, a transit or laser level is more appropriate.
Floor sagging beneath door jambs resulting from improper support below the jamb. This can be a structural concern. If needed, additional bracing can be added between the joists where the sag occurs, but with some difficulty if above a finished ceiling.Cracking in interior walls around openings, which may be caused by inadequate, deflected, or warped framing around the openings; differential settlement; or on the interior of masonry load-bearing walls, by problems in the exterior masonry wall. Cracking due to framing problems is usually not serious, although it may be a cosmetic concern that can be corrected only by breaking into the wall.
Sagging in sloped roofs resulting from too many layers of roofing material, failure of fire retardant plywood roof sheathing, inadequate bracing, or undersized rafters. Sometimes three or more layers of shingles are applied to a roof, greatly increasing its dead load. Or, when an attic story has been made into a habitable space or otherwise altered, collar beams or knee walls may have been removed. A number of factors, such as increases in snow and wind loads, poor structural design, and construction errors result in undersized rafters. Check for all these conditions.Failure of Fire Retardant Plywood (FRP) used at party walls between dwelling units in some townhouses, row houses, and multiple dwellings is not uncommon. Premature failure of the material is due to excessive heat in the attic space. On the exterior, sagging of the roof adjacent to a party wall often is evident. On the interior, check for darkening of the ply-wood surface, similar to charring, as an indication of failure that requires replacement of the FRP with a product of com-parable fire resistance and structural strength.
Spreading of the roof downward and outward due to inadequate tying. This is an uncommon but potentially serious structural problem. Look for missing collar beams, inadequate tying of rafters and ceiling joists at the eaves, or inadequate tying of ceiling joists that act as tension members from one side of the roof to the other. Altered trusses can also cause this problem. Check trusses for cut, failed, or removed members, and for fasteners that have failed, been completely removed, or partially disconnected. Spreading can be halted by adequate bracing or tying, but there may be damage to masonry walls below the eaves. It is possible that the roof can be jacked back to its proper position. Consult a structural engineer.Deflection of flat roofs due to too great a span, overloading, or improper support of joists beneath the roof. This is a common problem and is usually of no great concern unless it results in leaking and subsequent damage to the structure, or unless it causes water to pond on the roof, thereby creating unacceptable dead loads. In both cases, the roof will have to be strengthened or re-leveled.
A common location of fungal and insect infestation is where wood door frames touch concrete or earth at grade
The moisture content of properly protected wood structural components in buildings usually does not exceed 10 to 15 percent, which is well below the 25 to 30 percent required to promote decay by the fungi that cause rot or to promote attack by many of the insects that feed on or inhabit wood.
Dry wood will never decay. Inspect all structural and non-structural wood components for signs of fungus and insect infestation, including wood stains, fungi, termite shelter tubes, entry or exit holes, signs of tunneling, soft or discolored wood, small piles of sawdust or frass, and related signs of infestation.
Probe all suspect wood with a sharp instrument and check its moisture content with a moisture meter. Wood with a meter reading of more than 20 to 25 percent should be thoroughly examined for rot or infestation. Sound wood will separate in long fibrous splinters, but decayed wood will lift up in short irregular pieces.
Exterior building areas or components that should be checked are:
- Places where wood is in contact with the ground, such as wood pilings, porch and deck supports, porch lattices, wood steps, adjacent fences, and nearby wood piles.
- Foundation walls that might harbor shelter tubes, including tubes in the cracks on wall surfaces.
- Frames and sills around basement or lower level window and door frames, and the base of frames around garage doors.
- Wood framing adjacent to slab-on-grade porches or patios.
- Wood near or in contact with roofs, drains, window wells, or other areas exposed to periodic wetting from rain or lawn sprinklers, etc.
Interior areas or components to be checked for rot or infestation are:
- Spaces around or within interior foundation walls and floors, crawl spaces, piers, columns, or pipes that might harbor shelter tubes, including cavities or cracks.
- The sill plate that covers the foundation wall, and joists, beams, and other wood components in contact with it.
- Wood frame basement partitions.
- Baseboard trim in slab-on-grade buildings.
- Subflooring and joists below kitchen, bathroom, and laundry areas.
- Roof sheathing and framing in the attic around chimneys, vents, and other openings.
Damage to wood from fungal or insect attack usually can be repaired at a reasonable cost by replacing or adding supplemental support to affected support to affected components after the source of the problem has been corrected. Damage is rarely severe enough to seriously affect the structural stability of a building, although individual members may be badly deteriorated. Consult an exterminator when evidence of insect attack is found.
This fire-damaged wood should be carefully probed to determine the extent of charring. In this case, the wood was replaced.
When exposed to fire, wood first browns, then blackens, then ignites and begins to char at a steady rate. The charred portion of the wood loses its structural strength, but the clear wood beneath does not, unless it undergoes prolonged heating. The remaining strength of wood exposed to fire can be determined by removing the char and estimating the size and strength of the new cross section. Damaged structural members may be reinforced by bolting additional structural members in a configuration that restores their original design strength. Consult a structural engineer before repairing major structural beams or girders.
4) Connection failure and improper alteration
Open-web wood trusses are commonly used in small residential buildings as roof and floor structures. Those trusses with wood chords and wood webs usually use metal plate connectors. Where there is evidence of moisture, examine connectors for corrosion and for loss of embedment due to distortion of truss members. Also check glue laminated timber beams for delamination where there is evidence of moisture. Where possible, examine truss, rafter, and joist hangers for corrosion and proper nailing.
Buildings are often altered incrementally by the addition of pipes and ducts when unfinished spaces, such as basements and attics, are made habitable or when kitchens and bathrooms are remodeled. Observe joists, rafters, and beams for holes cut through them, especially for any size cutouts at or near their top or bottom. Observe wood trusses for cuts through chords and webs of open web trusses and webs and flanges of plywood trusses.
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